This invention relates to the low-temperature fractionation of air to obtain low-purity oxygen and nitrogen-rich products. The term "low-purity-oxygen" as used throughout the present specification and claims is intended to mean a product having an oxygen content of less than 99.5 mole percent.
It is believed that very large quantities of low-purity oxygen will be required by processes now being developed for converting coal to liquid or gaseous products. Another use for low-purity oxygen is in a process for converting refuse to useful gaseous products as described in Anderson, U.S. Pat. No. 3,729,298. Hence, a process for producing low-purity oxygen in large quantities at low cost is desirable.
A common system for low temperature fractionation employs a higher-pressure rectification column having its upper end in heat exchange relation with the lower end of a lower-pressure rectification column. Cold compressed air is separated into oxygen-enriched and nitrogen-rich liquids in the higher-pressure column, and these liquids are transferred to the lower-pressure column for separation into nitrogen-rich and oxygen-rich products. Examples of this double-column distillation system appear in Ruheman's "The Separation of Gases," Oxford University Press, 1945.
Large quantities of energy are required to compress the feed air for such a process. Hence, in these times of rising energy cost, a saving of energy is important. Coveney, in U.S. Pat. No. 3,731,495, discloses a system for reducing the energy required by the double-column distillation system by use of a nitrogen-quenched power turbine. A portion of the compressed feed air is mixed with fuel and combusted. The hot combustion mixture is then quenched with waste nitrogen-rich gas from the lower-pressure column, and the resulting gaseous mixture is expanded in a power turbine. The expansion provides energy to compress the feed air to the system. A disadvantage of the Coveney process is that the pressure of the gaseous mixture expanded in the power turbine can be no higher than that of the waste nitrogen mixed with the combustion gases. Hence, it would be impossible, in the Coveney process, to operate both lower pressure column and turbine at their respective optimum pressures, unless both had the same optimum pressure. However, it has been found that commercially available power turbines usually have optimum inlet pressures exceeding the optimum operating pressure of the lower-pressure rectification column in a typical air-separating system. This is true even for most of the higher-than-normal pressures used in the lower-pressure rectification column of the Coveney process. Hence, Coveney's invention is unable to achieve optimum operation of both the distillation system and the power turbine.
Another cryogenic air-separation system using a power turbine is disclosed by Swearingen, in U.S. Pat. No. 2,520,862. The Swearingen process mixes waste nitrogen-rich gas obtained from the higher-pressure column with a portion of compressed feed air. Fuel is then injected into the mixture, and the mixture is combusted and expanded in a power turbine, thereby providing energy to compress the feed air for the system. Like the Coveney process, Swearingen's process requires that the pressure of the gaseous mixture expanded in the power turbine be no greater than that of the nitrogen mixed with the combustion mixture. Hence, Swearingen is also unable to independently set the pressure of the turbine inert gas and higher pressure column to achieve optimum operation of both the power turbine and the distillation system. Swearingen, has a further disadvantage in that the nitrogen stream removed from the higher-pressure rectification column is unavailable for feeding to the lower-pressure rectification column, thereby depriving that column of reflux in proportion to the amount of nitrogen removed from the higher-pressure stage.